IntroductionImatinib mesylate, also known as STI571 or Glivec (Novartis, Basel, Switzerland), is a promising new treatment for chronic myelogenous leukemia (CML). Imatinib mesylate is a 2-phenylaminopyrimidine derivate that was designed as a selective competitive inhibitor of the Abl protein tyrosine kinases (v-Abl, Bcr-Abl, and c-Abl). [1][2][3][4][5] It also has strong activity against the platelet-derived growth-factor receptor (PDGF-R), c-Kit, ARG, and their fusion proteins Tel-Abl and Tel-PDGF-R, but does not affect other kinases. [6][7][8][9] Recent clinical trials of imatinib mesylate in the treatment of chronic-phase CML have demonstrated that the drug is well tolerated with only few adverse effects and can induce complete hematologic and cytogenetic responses in a significant proportion of patients. 2,3,[10][11][12] Moreover, activity of imatinib mesylate against more advanced, accelerated-phase blast crises and in patients with relapsed or refractory Philadelphia chromosomepositive (Ph ϩ ) acute lymphoid leukemias was reported. 13,14 Furthermore, in patients with gastrointestinal stroma tumors (GISTs), where activating mutations of c-Kit are likely responsible for the pathogenetic events, imatinib mesylate yielded encouraging results. 6,7,15,16 Because this tumor has so far been highly refractory to chemotherapy, imatinib mesylate is emerging as an important new therapeutic agent.However, the effects of imatinib mesylate on normal, nonmalignant hematopoietic cells have not been extensively evaluated so far. It is not clear whether some side effects like cytopenias that occur during treatment with imatinib mesylate may result from suppression of normal progenitor growth and differentiation. Recently, it was demonstrated that imatinib mesylate reduces the number of colony-forming cells in peripheral blood or bone marrow (BM) from patients with CML, with minimal effect on normal cells. 17 Dendritic cells (DCs) are recognized as the most powerful antigenpresenting cells (APCs) with the unique ability to initiate and maintain primary immune responses. They originate from BM-derived progenitor cells, spread via the bloodstream, and can be found in almost every organ as the sentinels of the immune system. In vitro, DCs can be generated from peripheral blood monocytes using granulocytemacrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). The differentiation of DCs from CD34 ϩ progenitor cells can be mediated by different cytokines like GM-CSF, tumor necrosis factor ␣ (TNF-␣), IL-4, and FMS-like tyrosine kinase 3 (FLT3) ligand and stem cell factor (SCF). [18][19][20][21][22][23][24][25] Because SCF has been shown to play an important role in DC development, we here explored a potential effect of imatinib mesylate on the development of mobilized human CD34 ϩ peripheral blood progenitor cells (PBPCs) into DCs. We show that in vitro exposure of CD34 ϩ PBPCs to therapeutic concentrations of imatinib mesylate (1-5 M) affects the differentiation and functional properties of generated DCs via inhibit...

Dendritic cells are the most powerful antigen-presenting cells playing a decisive role for the initiation and maintenance of primary immune responses. However, signaling pathways involved in the differentiation of these cells have not been fully determined. Imatinib is a novel tyrosine kinase inhibitor effective against Abl kinases, c-Kit, and plateletderived growth factor receptor. Using this compound, we show that human monocyte-derived dendritic cells generated in the presence of therapeutic concentrations of imatinib show a reduced expression of CD1a, MHC class I and II, and costimulatory molecules as well as decreased secretion of chemokines and cytokines resulting in an impaired capacity of dendritic cells to elicit primary T-cell responses. Using Western blot analyses, we found that these effects are mediated by inhibition of phosphatidylinositol 3-kinase/Akt pathways and a pronounced downregulation of nuclear localized protein levels of nuclear factor-KB family members. Importantly, using blocking antibodies and tyrosine kinase inhibitors, we show that the inhibitory effects of imatinib on dendritic cell differentiation are not mediated via platelet-derived growth factor receptor and c-Kit. Taken together, our study reveals that imatinib inhibits dendritic cell differentiation and function via Akt and nuclear factor-KB signal transduction. Importantly, we show that imatinib can inhibit the function of normal, nonmalignant cells that may result in immunosuppression of these patients.

Imatinib is a novel tyrosine kinase inhibitor effective against Abl kinases, c-Kit and platelet-derived growth-factor receptor (PDGF-R) and is currently used for the treatment of patients with CML and GIST. However, little is known about the effects of imatinib on function and differentiation of non-transformed normal cells. Using this compound, we show that human monocyte derived DC generated in the presence of therapeutic concentrations of imatinib show a concentration dependent reduced expression of CD1a, HLA and co-stimulatory molecules as well as decreased activation-induced secretion of chemokines and cytokines involved in T cell activation. Moreover, exposure to imatinib reduces the capacity of DC to prime T cell responses that cannot be restored by the addition of IL-12 and which is not due to induction of apoptosis or IL-10 secretion. Using Western blot analyses we found that these effects are mediated by a pronounced downregulation of nuclear localized protein levels of NF-kB family members RelB, RelA and NF-kB p50. Furthermore, imatinib treatment inhibited the phosphorylation of Akt, indicating the involvement of the PI3 kinase pathways while not affecting the phosphorylation state of p38 and ERK1 MAP kinase. In line with these results, incubation of monocytes with PI3 kinase inhibitors resulted in a similar phenotype of DC as described above. Gene expression profiling utilizing DNA microarrays revealed upregulation of lysosomal genes and molecules preferentially expressed in monocytes/macrophages. However, in contrast to these observations, imatinib treatment had no effect on the incorporation of latex beads by DC and resulted in a reduced FITC-labeled dextran uptake. Importantly, utilizing blocking antibodies and tyrosine kinase inhibitors we demonstrate that the inhibitory effects of imatinib on DC differentiation are not mediated by PDGF-R and c-Kit but most likely via c-Abl tyrosine kinase. These results demonstrate that imatinib affects the antigen presenting function of DC on several levels: their phenotype, antigen uptake and processing as well as production of cytokines and chemokines.